Determining R/S on a Fischer Projection

Hi! Can you please explain, if there is a longer chain, eg 3 stereocentres, how do you know for the middle stereocentre how to draw it in 3d? Which way is ‘up’ (flat on the page) and which way would be ‘down) (ie going behind us)

Always for a Fischer, the groups pointing on the side are coming out of the page. That’s all the information you need to figure out R/S. As far as drawing in 3D, if you have that information, it’s probably best to redraw the molecule in “zigzag” formation and then use the R/S information to draw in the stereocenters that way.